Among various fillers used for rubber compositions, silica has drawbacks in processability although silica provides a small rolling resistance and excellent braking property and steering stability on wet roads. For example, it is necessary that a multi-stage mixing be conducted due to a great viscosity in the uncured condition. Silica has other drawbacks in that dispersion of the filler is poor, vulcanization is delayed, and strength at break and abrasion resistance are markedly decreased (for example, Japanese Patent Application Laid-Open No. Heisei 8(1996)-176345).
When silica is mixed with rubber, it is widely conducted to overcome the problems that a coupling agent is added so that the viscosity in the uncured condition is decreased and the modulus and the abrasion resistance are improved. However, this process has a problem in that the coupling agent is expensive, and the cost of production increases depending on the formulation.
It is also conducted that an additive for improving dispersion is used to improve the processability by decreasing the viscosity in the uncured condition through improvement in the dispersion of silica. However, this process has a drawback in that abrasion resistance decreases. When a strongly ionic compound is used as the agent for improving dispersion, processability such as adhesion to rolls occasionally decreased.